Apple's AAC encoders are often touted as being the 'best', quality-wise: https://hydrogenaud.io/index.php/topic,119876.0.html

As jammmety said; for AAC encoding yes, but don't worry - you can have ffmpeg use that encoder to get the best of both.

afconvert is the only way I've managed to turn mp3s into something that Books would accept as an audiobook.

    afconvert -v -s 3 -f m4bf ....mp3